I had originally planned to post a great deal of material on Purim. Unfortunately, the exigencies of practical life over the past two weeks prevented that from happening. Be that as it may, I'd like to offer a few basic thoughts about the nature of Tanach in lieu of a more extensive presentation on the themes of Purim in particular.
It is well known that "Tanach" is not a monolithic entity. Its contents varied over time and were reconsidered and adjusted at several points in Rabbinic history. The Talmud tells us, for example, that the Rabbis debated whether or not to keep the books of Ezekiel and Ecclesiastes in the canon. They also argued about the precise status of some of the canonical books, such as the Book of Esther.
Many find this phenomenon troubling. After all, if a text is holy and presumed to be divinely inspired, why should it be excluded from Tanach? And if the holiness of the work itself is what is being questioned, how can logical argumentation in the Talmud serve to establish it? Jewish law cannot "rule" on an empirical fact, such as whether divine guidance played a role in the composition of a particular book!
We can sharpen our question further by asking what difference it makes whether a book is included or excluded from the canon. For example, did the Rabbis who maintained that the Book of Esther was not part of Tanach believe that reading Esther was a waste of time? Did the Rabbis that advocated removing Ecclesiastes or Ezekiel from the canon hold that studying these books would not be meaningful, or that their content was inaccurate or false? It is hard to accept such a conclusion, especially since, in the case of the Book of Esther, even those who held it was not part of the canon still agreed that it was a mitsvah to read it on Purim.
Study of the nature of Tanach leads us to another interesting problem. The Mishna in Masechet Megillah discusses the guidelines that must be observed in the preparation of Torah Scrolls, Megillot and books of the Prophets and Writings (Nach). All of these texts must be written on kosher parchment with kosher ink, etc. The difficulty is as follows: We know that there is a mitsvah to write a Torah scroll. Similarly, Megillot must be written in order to be used for the mitsvah of reading the Megillah on Purim. Therefore, it makes sense that there are halachic principles that dictate the mechanics of composing these texts.
However, there is no mitsvah at all to read most of the books of Tanach, at least not publicly. Being that there is no commandment to read from these volumes, and therefore no mitsvah to write them, how can there be halachic guidelines as to their preparation?
Indeed, the Rabbis themselves seemed ambivalent about the value of Tanach. Several statements of Chazal declare that the entirety of the Bible was given at Sinai, implying that its importance is on par with that of the Torah itself. Other statements suggest that the Prophets and Writings will be eliminated in the Messianic era, and that only the Torah and the Book of Esther will remain (this view is in fact codified by Maimonides at the end of Hilchot Megillah). How can we reconcile these contradictory perspectives on the Bible?
In order to resolve these difficulties, we must address the most basic question of all: Why does Tanach exist in the first place? We can understand the need for Torat Moshe, which provides us with a theological framework and a system of commandments to abide by. But what purpose is served by additional holy books?
I would suggest that the ultimate goal of all Jewish learning is the proper comprehension of Torah, i.e., the Five Books of Moses. It is the Torah of Moshe that includes, explicitly or implicitly, all of the ideas that comprise God's prophetic message to the people of Israel. However, bringing out the latent content of the Torah is no simple matter. In fact, its true meaning can be obscured by a variety of factors, such as the intellectual ability of its students, the influence of current cultural trends, etc. It is precisely to form a "bridge" between the comprehension of a given generation of Jews and the ideational content of the Torah that the Nach comes into existence. The books of the Prophets and Writings revolve around themes that are present in the Torah in some form but were deemed by the Baalei Hamesorah to require a "fleshing out", a separate "academic course" dedicated to them in their own right.
Examples of this kind of phenomenon abound in secular culture. Despite the bold proclamation that "all men are created equal", our country tolerated slavery and discrimination against women for hundreds of years. Many tracts were written detailing the incongruence and hypocrisy inherent in this state of affairs, until it was finally comprehended by the common man and a shift in cultural attitudes ensued. Nowadays, most of those important texts have become obsolete historical relics. They are no longer studied in depth because their message has already been internalized by the average person, who perceives their truths naturally in the simple words "all men are created equal".
Similarly, the Torah teaches the ideal of a wise life of prudence and justice. This is implicit in the Torah's narratives and commandments, and, in the proper cultural context, this overarching principle would be obvious to its readers. Nevertheless, King Solomon saw fit to author two books - Ecclesiastes and Proverbs - in which he expounded upon these themes at length. He understood that a separate "curriculum" was necessary for these ideas alone, and that such a study had to be completed in order for the average person to grasp the true import of the Torah's lessons.
Thus, the concept of Tanach is not a differentiation between meaningful/inspired books and meaningless or secular ones. Rather, a book is "inducted" into Tanach when the Baale Mesora determine that, if the Torah is to be understood properly, this additional book must have a separate course of study dedicated to it as well.
In this sense, becoming a book of Tanach is more a function of the laws of Torah Study than of a particular book's intrinsic value. A divinely inspired book may not demand a separate activity of learning and analysis just by virtue of its holiness. In some cases, it may be excluded from Tanach because the Rabbis think it will interfere with the correct understanding of Torah in their generation. Similarly, a non-inspired book like Proverbs may still be seen as an indispensible "course" in the Talmud Torah curriculum, despite its lack of "holiness".
The fact that these texts demand a process of learning in their own right is reflected in the halachic principle that they must be committed to writing in the same fashion as a Torah Scroll. They become an additional component of the Written Torah; therefore, although there is no specific commandment to write them, when they are written it must be done in a manner that demonstrates their special status.
What, then, is the status of a holy book that is rejected from Tanach? I would argue that, rather than robbing it of importance or significance, this merely relegates its contents to the status of Oral Torah, of useful commentary that may shed light on the meaning of other books in the canon. The Book of Esther is an excellent example of this. It is dedicated to a theme that is repeated numerous times in Tanach - the struggle with Amaleq. If it had not been officially accepted as part of the canon, it would have remained a very important, divinely inspired addendum to the presentations of this theme elsewhere in the Bible. It would have been read and studied, not as a course in its own right, but as a source of clarification and elucidation of the concept of Amaleq that is mentioned in the Torah and in the Book of Samuel. And, of course, it would have remained the central focus of the Purim celebration!
This also clarifies how the Rabbis could debate the inclusion or exclusion of particular works from Tanach. They were not engaged in arguments about the value of those texts, or their status vis a vis divine inspiration. What concerned the Sages was whether those books were necessary "courses" in the Torah curriculum of their generation. In some cases, the analysis revolved around whether involvement in certain texts would prevent people from attaining true knowledge of the Torah and its commandments. Whatever the case might have been, the Rabbis never questioned the accuracy or validity of any of the holy books in their possession. The issues they grappled with were halachic, not theological or empirical.
This approach is supported by the Rambam who, in his Laws of Torah Study, emphasizes that the Prophets and Writings are considered parts of the Written Torah. It is noteworthy that he establishes this classification in the context of the laws of learning Torah - and, specifically, while dealing with the correct Torah study schedule. This suggests that the distinction between Written and Oral Torah has more to do with the proper allocation of study time than with the intrinsic importance of the subject matter assigned to each category. Only texts that generate their own, independent obligation of Torah study are granted membership in the canon of Tanach. Other books, however profound and beautiful, are to be utilized as interpretive tools in the process of exploration of Biblical literature.
This is why the Rabbis can simultaneously claim that the entire Tanach was given at Sinai, and then state that all of the Prophets and Writings, save Esther, will one day be obsolete. The thematic principles of Tanach are all rooted in the Torah itself, they derive from Sinai. However, our need for separate courses of study to elucidate and deepen our grasp of these principles - that is, the fact that we cannot access them directly through study and interpretation of the Five Books of Moses - is a function of our spiritual weakness at this point in history. In Messianic times, the level of Torah study will be such that no other "Books" will be necessary - we will have Torat Moshe alone, and all other writings will serve as commentary.
In summary, aside from the Torah, the composition of Tanach is not divinely established, and inclusion or exclusion from Tanach is not a reflection of the truth or importance of any particular book. The structure of Torah literature - what attains the status of Written Torah and what does not - is purely a function of the educational needs of each generation, as determined by the Masters of the Torah Tradition. This determination impacts the process, order and format of Torah study, but not necessarily its scope.